1. Field of the Invention
The present invention relates to a method for driving a liquid crystal display, and more particularly, to a driving method which enables the after-image phenomenon to be reduced.
2. Discussion of the Prior Art
Recently, active matrix liquid crystal displays are widely used in which switching elements such as thin film transistors (TFTs) and pixel electrodes are arranged in a matrix. However, since the liquid crystal display is capacitive, it has a hold type luminous characteristic, and data once written into a pixel is held until it is rewritten after one frame period. Thus, it has a problem that after-image is conspicuous as compared with the inpulse type luminous display such as the CRT display, which instantaneously emits light within one frame period, and in particular, the display characteristics degrade when displaying a motion picture.
As a countermeasure for the after-image problem, Japanese Published Unexamined Patent Application No. 64-82019 proposes to control backlight. The illuminating equipment for backlight consists of an array of a plurality of lamps, which are sequentially turned on and off according to the timing of line scanning of liquid crystal display. Each lamp covers a group of a predetermined number of (e.g. 44) scanning lines. Each lamp lights up when all the scanning lines of the associated group are driven, and goes out after the elapse of a fixed time. However, in this case, since blanking (erasure of a display) is performed on a group-by-group basis, there is a problem that blanking cannot be controlled for each scanning line.
To solve the problem of the Japanese Published Unexamined Patent Application No. 64-82019, U.S. patent application Ser. No. 150,975 filed on Sep. 11, 1998 and entitled xe2x80x9cA METHOD OF DISPLAYING AN IMAGE ON LIQUID CRYSTAL DISPLAY AND A LIQUID CRYSTAL DISPLAYxe2x80x9d assigned to the assignee of the present invention discloses a method in which the liquid crystal panel is divided into the upper half and the lower half. The liquid crystal panel is controlled to simultaneously drive a pair of gate lines (one from the upper half and one from the lower half). The upper and lower half gate lines are sequentially driven line by line as a pair to display data for one frame in a predetermined period of one frame period (for instance, the first half of one frame), and in the remaining period of one frame (for instance, the latter half of one frame), they are sequentially re-driven line by line as a pair to forcedly write a blanking image (black image). This method is to shorten the lighting time or display time by forcedly writing black in the same frame period, whereby the after-image problem can be properly solved. However, the liquid crystal panel needs to be divided into two, and a special gate line driver circuit for simultaneously driving the panel halves and two data line driver circuits for independently driving each panel half are required, which leads to a problem that the panel structure and the driver circuits become complicated. Further, since one frame period is divided into two, the first one assigned to displaying and the second one assigned to blanking, the blanking time cannot be changed without changing the display time. Accordingly, there is a problem that the blanking time cannot be freely set without affecting the displaying of an image.
Accordingly, it is an object of the present invention to provide a method for driving a liquid crystal display, which can advantageously solve the after-image problem by controlling the blanking on a scanning line-by-scanning line basis, without requiring any special panel structure or driver circuit.
The present invention is a method for driving an active matrix liquid crystal display having thin film transistors and pixel electrodes at the intersections of gate lines receiving a scanning signal and data lines receiving a data signal, each pixel electrode and an adjacent gate line forming an auxiliary capacitor. The driving method of the present invention comprises a step of writing data into the pixel electrode sequentially line by line in response to the scanning signal and the data signal to display an image for a frame, and a step of driving the gate lines sequentially line by line at a predetermined time before the period of a frame ends to control the potential of the pixel electrodes through the auxiliary capacitors thereby for forcedly blanking a display. Preferably, the blanking is performed by writing a black level, and the auxiliary capacitor is formed by the associated pixel electrode and the preceding gate line.
The after-image problem can advantageously be solved by controlling the blanking on a scanning line-by-scanning line basis, without requiring any special panel structure. Further, optimization can be made by freely setting the blanking time, without affecting the image display.